1. Technical Field
The present invention relates to systems and methods for reproducing a television signal on a reproduction device, and in particular, to a mobile television receiver providing a television signal for reproduction to a reproduction device.
2. Related Art
Automotive vehicles are increasingly being equipped with on-board television receivers. The television receivers may receive television programs broadcast terrestrially by one or more radio stations. The radio stations may be distributed such that, at each vehicle location within an area covered by a broadcast program, the vehicle can receive the program by at least one of the radio stations. Television receivers used in vehicles may select a particular channel for receiving radio waves of a nearby radio station, wherein the channel selection depends on the driver's program selection and the vehicle position in relation to the closest radio stations. Presently-used radio chains are operated such that each radio station transmits the television signal on an individual channel, i.e., an individual radio frequency. During travel, the received television signal at the mobile television receiver may fade, necessitating the television receiver to continuously select a radio station providing a better signal quality, i.e., having a higher signal strength.
The terrestrially broadcast television signals are typically transmitted in an analog manner based on long established television standards. Examples of analog television standards include PAL, SECAM and NTSC. The PAL (Phase Alternate Lines) standard is typically used in western Europe and Asia. The SECAM (Sequentiel Couleur Avec Memoire) standard was developed based on PAL and is in use, for instance, in France, Eastern Europe and Northern Africa. PAL and SECAM use the same resolution of 625 lines at a frame rate of 25 frames per second. The NTSC (National Television Standards Committee) standard is used in Northern America and Japan. NTSC has a lower line resolution compared to PAL and SECAM (525 lines) and displays the frames at a frame rate of 30 frames per second.
The television signal received by a television receiver may first be decoded to a composite video signal (CVS). Typically, the composite video signal is converted to YUV or RGB signal and directly applied to a display device. Television signals may also be broadcast in the form of digital data. Examples of digital televisions standards include digital video broadcasting (DVB) or digital audio broadcasting (DAB).
Mobile television receivers may be used in any mobile device, including any vehicles, coaches and trains. In these vehicles, one or more display devices may be mounted at separate, distant locations from the television receiver. The transmission of video data from the television receiver, which is intended for display on a display device, is accomplished by employing a particular communication link connecting the television receiver and the display device.
Today's vehicles are increasingly being equipped with a communication network for transmitting entertainment and information data within the vehicle. Such a communication network may also be used to transmit the video signal from a television receiver for display on a display device. Examples of communication networks include the IEEE 1394 standard and the MOST standard. MOST communication networks are particularly intended for use in vehicles.
For transmission of the video data on such a communication network to one or more display devices, the video information to be transmitted may be adapted to the transmission scheme employed on the communication network. For this purpose, the video information may be transmitted in accordance with one of the standardized digital video encoding techniques. Such digital coding standards include, for example, the MPEG-x standards (MPEG-1, MPEG-2, MPEG-4 and MPEG-7) and the H.26x standards (H.261, H.262, H.263 and H.264).
In particular, MPEG-2 standards define the format of various component parts of a multi-media program. Such programs may consist of MPEG-2 encoded video, audio, control data, and so forth. MPEG-2 further defines the multiplexing of the components into a single synchronous transmission bit stream. The most basic component of an MPEG bit stream is the “elementary stream.” Each elementary stream output by MPEG audio, video or data encoders contains a single type of signals. Each elementary stream is accumulated into a stream of “Packetised Elementary Stream” (PES) packets. A PES packet may be a fixed or variable sized block. The PES includes synchronization information such as the “elementary stream clock reference” (ESCR).
To generate an MPEG transport stream (TS), each PES packet may be broken into fixed-size transport packets forming a general purpose way of combining one or more streams. The transport stream may comprise a sequence of packets of 188 bytes. Each packet may include 184 bytes of payload and a 4 byte header.
The 4 byte header includes a packet identifier (PID), which may uniquely identify the stream to which the packet belongs and allow a receiver to respectively differentiate the TS packets. Each packet is associated with a PES through the setting of a PID value in the packet header. Some PID values are predefined and are used to indicate various streams of control information. A packet with an unknown PID may be silently disregarded by a receiver. A particular PID value of 0x1FFF is reserved to indicate that the packet is a null packet and to be ignored by the receiver. Such null packets are inserted to retain the specified TS bit rate.
Audio and video data may be transmitted to a reproduction device over a communication network in form of a MPEG TS. The MPEG TS includes audio packets and video packets differentiated by different PID values. Typically, there are more video than audio packets to be transmitted.
A reproduction device may include a display for displaying the video signal and have loudspeakers associated thereto for reproducing the audio signal. For transmission to the reproduction devices, the television signal may be encoded in accordance with one of the digital encoding standards, such as MPEG TS form, and transmitted towards the reproduction devices (also called “head units”). Each reproduction device may be provided with a decoding unit, which decodes the received encoded audio and/or video signals into analogue signals for reproduction on the display screen or the loudspeakers.
During travel of the vehicle, the signal strength of the received television signal varies and tends to undergo fading. Such variations in signal strength may cause distortions in the reproduced image on the display device and also in the audio signal. For effectively suppressing a disturbance in the recovered images and audio data, the television receiver has to continuously search for and switch to a television signal having a better signal strength.
The occurrence of fading and a switching between different radio stations continuously interrupts the received television signal and causes distortions in the reproduced audio and video signal. Depending on the length of each of the distortions, the digital encoding procedure and the transmission of data to the reproduction device is interrupted. Such interruptions cause the encoder to stop the encoding operation. As a result, the sequence of encoded video frames and audio data received at the reproduction device is abruptly stopped. A time consuming initialization procedure is required in the decoder of the reproduction device when the signal distortion is terminated and the transmission of encoded video and/or data is resumed. Thus, the quality of the reproduced television signal is more greatly affected, since the actual period of the distortion is extended due to a required resynchronization procedure at the display device.
The length of the interruption of the video signal display on the display device is prolonged by all steps needed to restart the decoding and reproduction procedure. For instance, each decoding unit is provided with a buffer to prevent a data underflow or overflow of the received audio and/or video data. The video buffer is operated such that the video data level is maintained around a predefined buffer level value. Consequently, a decoding and reproduction of new video information at the reproduction device cannot be started until the buffer has been filled again up to the predefined buffer level. Hence, the disturbance period on the display is affected accordingly.
Typically, to reduce the impact of signal distortions on the perceived image quality, error concealment occurs at the display side, i.e., within the reproduction device. The time period, when the reproduction device does not receive any new encoded video information, is bridged by applying predetermined video information to the display screen within the display device. This is accomplished by an additional memory provided in the display device storing video information in advance for reproduction on the display screen. Such video information may be, for instance, a screen of a uniform color or a uniform background having a text, such as “no signal,” inserted therein.
Another typical approach for masking the video signal interruption at the display device is a freezing of the last completely received and encoded video frame and repetitively reproducing same until new video data is received at the display device.
Therefore, a need exists for a television receiver and methods for reproducing a television signal that provide an improved quality of the reproduced signal.